Despite what Disney cartoons and breakfast cereals may have taught us, animals cannot talk. This is a more profound statement than it at first appears.

It isn’t just that they can’t make the vocal noises required to form words. There’s some evidence that speech and intelligence are intimately related, that it’s only language that allows us to formulate abstract ideas. There is a reason why your dog is quite capable of telling you it’s hungry, but has yet to reminisce about that walk in the park last Sunday.

Columbia University linguist Herbert Terrace thought all this worth investigating. So, in 1973, he came up with an experiment. He’d take a baby chimp from its mother, give it to a human family and teach it to communicate using sign language. Thus we’d find out what facilities our nearest relatives, at least, have to express themselves. The animal in question he named Nim Chimpski, a joke at the expense of a better known academic rival.

James Marsh’s ‘Project Nim’ uses a combination of interviews, archive footage and reconstructions to document the experiment in all its lunacy and dodgy ethics, and its story is disturbing on a number of levels. For one thing, Nim’s progress from rural idyll to medical lab is oddly reminiscent of Kazuo Ishiguro’s ‘Never Let Me Go’.

Just when you think it’s all going to end in tragedy, though, there’s a twist straight out of ‘Miracle on 34th Street’, and it turns to farce instead.

Then there’s the fact that most of the key players in this story are, let’s be blunt about this, nuts. Terrace is a shrivelled, moustachioed lump of self-justification, and seems to have a moral code less evolved than that of his subject. (Nim, at least, knows the word ’sorry’.)

His adoptive ‘mother’ Stephanie LaFarge is little better, bringing a chimp into a house of seven children on the Upper West Side and – this is the upsetting part – breast-feeding him. Later, after a decade apart, she visits Nim in his new home, and tells us “he wasn’t particularly attractive to me now he was an adult chimp”. It all raises rather a lot of unpleasant questions.

This blurring between Nim’s ‘human’ and animal natures is the most fascinating and uncomfortable thing about the entire venture. He wears clothes, knows what you do with shoes, drinks booze and smokes pot. He can tell you what he wants, how he’s feeling, and, when he tires of his lessons, that he needs to use the toilet. Some of his carers clearly saw him as more child than animal, bitterly accusing Terrace of deceiving him and weeping copiously at the memory of his story.

But – he’s a chimp. He’s capable of sudden, horrible violence, coming close to maiming at least two of his carers. At one point, he asks someone to pass him a cat because he fancies a cuddle. He tries to have sex with it.

If the experiment raises questions about Nim’s dual natures, though, Marsh’s film does the same for its human players. They’re cruel and abusive, they fight each other for dominance, they take a baby from its mother even as both scream. Terrace and LaFarge alike deny their lusts had any part to play in their actions, and we don’t believe either one of them. Animals may not all be human, but all humans are undoubtedly animals.